Crossover from BCS-superconductivity to Bose-condensation

Starting from a model of free Fermions in two dimensions with an arbitrary strong effective interaction, we derive a Ginzburg-Landau theory describing the crossover from BCS-superconductivity to Bose-condensation. We find a smooth crossover from the standard BCS-limit to a Gross-Pitaevski type equation for the order parameter in a Bose superfluid. The mean field transition temperature exhibits a maximum at a coupling strength, where the behaviour crosses over from BCS to Bose like with corresponding values of 2 Δ₀/T_c ≈ 5 which are characteristic for high T_c superconductors.     

An extension of the Borel-Weil construction to the quantum groupU q (n)

The Borel-Weil (BW) construction for unitary irreps of a compact Lie group is extended to a construction of all unitary irreps of the quantum groupU _q(n). Thisq-BW construction uses a recursion procedure forU _q(n) in which the fiber of the bundle carries an irrep ofU _q(n–1)×U _q(1) with sections that are holomorphic functions in the homogeneous spaceU _q(n)/U _q(n–1)×U _q(1). Explicit results are obtained for theU _q(n) irreps and for the related isomorphism of quantum group algebras.Supported in part by the National Science Foundation, No. PHY-9008007     

The double duals and gauges of the Lanczos tensor

It is shown that: i) the Weyl tensor can be expressed in terms of the sum of a tensor and its double dual, where the tensor is constructed from the covariant derivatives of the Lanczos tensor, ii) a similar expression does not exist for the Riemann tensor in electromagnetic theory, iii) the electromagnetic field cannot be identified with the differential gauge freedom of the Lanczos tensor, iv) the symmetries of Einstein Maxwell theory and the Lanczos tensor do not prohibit the identification of the electromagnetic field with the algebraic gauge freedom of the Lanczos tensor, these symmetries require a differential equation relating the electromagnetic field tensor to the algebraic gauge vector and this is given.     

Single LED based dual wavelength referenced optical fibre sensor system using intensity modulation

An intensity-modulated optical fibre sensor system is described which employs a single LED source to provide the measurand and the reference signals with two separate wavelength bands. The allocated wavelength bands are selected to minimize any differential intensity effects that may arise from LED thermal variations. The sensor system, comprising a transceiver unit connected to an optical displacement sensor using a single optical fibre, provides an output fully referenced for all major common-mode variations that are likely to occur. Performance characteristics for the prototype system are reported which show a linear displacement range of over 20 mm.     

Asymptotics of a multiple Cauchy integral with rapidly oscillating exponential

An asymptotic expansion uniform with respect to a parameter is obtained for a multiple Cauchy integral with a rapidly oscillating exponential in the integrand under the assumption that the singularity lines are in a general position.Translated fromMatematicheskie Zametki, Vol. 58, No. 2, pp. 231–242, August, 1995.The work was financially supported by the Russian Foundation for Basic Research under grant No. 94-01-00193-a.     

Photoionization of impurities in heterojunctions

The spectral dependence of the photoionization cross-section is calculated for shallow donors in GaAs/GaAlAs heterojunctions. The resulting spectra is found to be different for light polarized along and perpendicular to the growth direction. The calculations are repeated for both infinite- and finite-barrier models.     

Reduction of tympanic membrane displacement during vocalization of the arboreal frog, Eleutherodactylus coqui.

Tympanic membrane (TM) velocities of vocalizing male frogs, Eleutherodactylus coqui, were measured in the field using laser vibrometry. The animal's call produced levels of 114-120 dB SPL measured at distances of 12-20 mm from the eardrum and mean TM displacements in response to the two notes of the male's own advertisement call were 1.84 and 2.08 nm, respectively. These values are close to the TM displacements obtained in response to playback of the two-note call at much lower sound levels. Thus TM vibrations are attenuated during phonation in the frog, and several candidate mechanisms for auditory sensitivity reduction are discussed.     

Smectic liquid-crystalline phases of F(CF2)7(CHOH)(CH2)8H and F(CF2)9(CHOH)(CH2)10H Comparison with other partially fluorinated molecules

Partially fluorinated alcohols F(CF₂)_m(CHOH)(CH₂)_nH (where m = 7, n = 8 and m = 9, n = 10) exhibit highly ordered smectic liquid-crystalline phases, as confirmed by optical microscopy and differential scanning calorimetry. Miscibility studies show that the smectic phases of the two alcohols are not of the same type. The related partially fluorinated ketones F(CF₂)_mCO(CH₂)_nH do not form a detectable mesophase. This is rather surprising since the (more polar) corresponding alcohols and the (less polar) analogous iodides and n-alkanes do. A brief discussion of the results and a comparison among partially fluorinated alkanes with various functional groups are presented.